(1) Field of the Invention
This invention relates to a fine silica tube composed of silica gel or silica glass, and a process for producing the fine silica tube composed of silica gel or silica glass, by a sol-gel process.
(2) Description of the Related Art
As inorganic fibers, those which are composed of carbon, alumina, zirconia, silica, potassium titanate and glass, are known. As whisker, those which are composed of carbon, alumina, silicon carbide and silicon nitride, are known. As fine tubes, those which are composed of carbon [see S. Iijima, Nature 354, 56 (1991)], boron nitride [see Ewan J. M. Hamilton et al., Science 260, 659 (1993)], and silica glass, are known.
It has been reported that, when powdery quartz and metallic silicon powder are treated at a temperature of 1,200.degree. C. and a reduced pressure of 10.sup.-3 to 10.sup.-4 mmHg, small amounts of fibrous and tubular materials are produced [see Th. Nemetscek and U. Hofmann, Z. Naturforsch 8b, 410 (1953) and Z. Naturforsch 9b, 166 (1954)]. These fibrous and tubular materials are composed of amorphous SiO.sub.2 containing metallic silicon.
Where the conventional silica or silica glass fibers are used as a filler or a reinforcer, it is eagerly desired to render them light-weight, enhance the flexural strength or render them hollow-shaped.
For the stable production of the conventional silica glass tube, problems arose in technology and an equipment.
Where the silica or silica glass fibers and tubes are used as a semiconductor material, it is especially required to enhance the purity, reduce the .alpha.-rays radiation and reduce the uranium content. However, these requirements are not satisfied. Where the silica or silica glass fibers are used as an implant material, similar requirements must be satisfied.
Further, a fine silica gel tube having a special shape and a high specific surface area is required where the silica gel tube is used for a catalyst carrier and an adsorbent.